Vacuum circuit interrupters



1962 J. M. LAFFERTY 3,016,436

VACUUM CIRCUIT INTERRUPTERS Filed July 24, 1958 HIGH MELT/N6 PU/N TINTEKMETALLIC COMPOUND C UKKE N T fnven'or': Jam s M .Ld er'ijy,

His Attorney.

3.616.436 VACUUM CKTMJUIT INTERRUFTERS James M. Lafferty, Schenectady,N.Y., assignor to General Electric Company, a corporation f New YorkFiled duty 24, 1958. Ser- No. 75%,615 13 Claims. (ill. zen-144 Thepresent invention relates generally to vacuum-type electric circuitinterrupters uniquely adapted for use in inductive circuits wherein thecurrent interrupted is of low magnitude.

The phenomena of electric current interruption in at mospheric or higherpressures, on one hand, and in high vacuum, on the other hand, areextremely complex and basically different in nature. So also, are theproblems which must be overcome in producing operative devices toperform either function. Thus, for example, one great problem innon-vacuum circuit interrupters is the deterioration of the contactsthereof due to the formation of oxides of the contact metal and otherchemical com pound due to the presence of atmospheric air, a protectivegaseous atmosphere, or an oil bath. Any compound so'fo-rmed is readilyseparated from the interrupter contacts. Such a continuous formation andremoval of nonmetallic compounds rapidly causes the interrupter contactsto erode and deteriorate.

In the vacuum interrupter, on the other hand, the complete absence ofgases orliquid in contact with the interrupter contacts virtuallyprecludes the formation of contact-eroding oxides and compounds.

Other problems, however, not generally considered or significant in thenon-vacuum circuit interrupter, are of prime importance in theconstruction of vacuum circuit interrupters. Vacuum interruptersdesigned to interrupt alternating currents operate substantially asfollows:

When an arc is struck between the electrodes of a vacuum interrupter, asfor example, by the opening of a pair of switch contacts maintained in avacuum of mm. of mercury pressure or less, the arc vaporizes some of theelectrode material. If the contacts have been properly processed toremove therefrom all occluded and sorbed gases, only ionized metallicvapor is present in the arc. The are continues until the alternatingcurrent sought to be interrupted falls to a natural zero value, asnormally occurs once each alternation. When this occurs, the arc isextinguished, and the ionized metallic particles rapidly diffuse to thecold vacuum chamber walls where they are cooled and tie-ionized. Whenthe interrupted current seeks to rise again, the arc cannot bereestablished because of the high dielectric strength of the vacuumseparating the interrupter contacts. Thus, in a vacuum circuitinterrupter, the current is usually completely extinguished at the firstnaturally occurring instantaneous zero value of current.

The foregoing occurs satisfactorily in vacuum circuit interruptersoperating to interrupt alternating currents of high current ratings,namely of 500 amperes or greater. For lower values of alternatingcurrent, the phenomenon generally denominated as chopping occurs. Atthese low alternating current values, the arc, rather than persistinguntil the first natural current Zero, is abruptly extinguished at somelow current value and falls instantaneous- 1y from that value to zero.The value of current at which instantaneous arc extinction occurs isdenominated the chopping current of the device. Typical choppingcurrents in prior art vacuum interrupters may be from 10 to 40 amperes.While chopping in vacuum interrupters associated with capacitivecircuits may be tolerated, it is unacceptable with inductive loadsbecause of voltage surges induced therein by the high rate of change ofcur- Patented Jan. 9, 19fi2 p ice rent with time (di/dt) when aninterrupter chops. This may be seen from the relationship:

V=the surge voltage induced by chopping I =the chopping current L=theequivalent inductance of the circuit C =the equivalent capacitance ofthe circuit,

and the fact that surge impedences for many inductive devices may be ashigh as many tens of thousands ohms.

For most industrial inductive circuit loads, it i neces sary to reducethe chopping current level of a vacuum circuit interrupter to a value ofbelow 4 amperes and, in most instances to a value of below 2 amperes.

Accordingly, it is an object of the present invention to provide vacuumcircuit interruptcrs suitable for interrupting low value alternatingcurrents in inductive circuits.

A further object of the invention i to provide a vacuum circuitinterrupter capable of reducing the value of instantaneous alternatingcurrent changes upon interruption thereof to a value of several amperes.

Still another object of the invention is to provide vacuum circuitinterrupters havingarc electrodes which minimize chopping and havehardness and brittleness characteristics which avoid permanentwelding-together of the contacts or shattering thereof.

In accord with the present invention, a vacuum circuit interrupter isprovided in the form of an evacuable chamber capable of maintaining avacuum of .less than 10" mm. of mercury, which chamber contains a pairof opposed arc-electrodes adapted to be the terminal points of anelectric are carrying the current to be interrupted. The portions of thearc electrodes which serve as such terminal points are substantiallyfree of occluded and sorbed gases and are formed primarily of anintermetallic compound having a melting point in excess of 800 C. andcomprising elements which are stable in air and may be convenientlyvacuum-treated to remove gaseous impurities. At least one of theelements comprising the compound utilized is selected to have a vaporpressure less than that of cadmium and greater than that of lanthanum.In a preferred embodiment of the invention, the chosen compound. alsocomprises a metal having a low work function.

The novel features believed characteristic in the present invention isset forth in the appended claims. The invention itself, however,together with further objects and advantage thereof, may best beunderstood by reference to the following, description taken inconnection with the attached drawing in which:

FIG. 1 is a representative view of a vacuum-type circuit interrupterconstructed in accord with the present invention and, 7

FIG. 2 is a graphical representation of asinusoidal alternating currentillustrating the effect of the chopping phenomenon.

In FIG. 1 an interrupter chamber 10 comprises a wall member 11 which maybe cylindrical in shape and is constructed of a suitable insulatingmaterial, having at the ends thereof a pair of metallic end members 12and 13 closing the volume therein to form an interrupter chamber,Suitable seals 14 are provided between casing 11 and end members 12 and13 to render the interrupter chamber vacuum-tight.

Located within chamber 10 are a pair of separable contacts orarc-electrodes 15 and 16 shown in their closed circuit or engagedposition. Upper contact 15 is a sta' tionary contact suitably attachedelectrically and mechanically to a conducting rod 17 which, at its upperend, is

united electrically and mechanically with end member 12. Lower contact1d, mounted upon and electrically united with a suitable conducting rod18, is movable and is connected through bellows 20 or an equivalentvacuum tight member permitting reciprocating motion. Terminal I mount 18projects through a suitable orifice in end member 13, and suitableactuating means may be connected thereto to impart reciprocating motionto rod 18 to cause contact 16 to enter into engagement with, and beremoved out of engagement With contact 15. For certain types ofinterrupters, as for example, vacuum fuses and lightening arresters,electrode 16 need not be movable but may be spaced apart from electrode15 a suitable distance. The electrical circuit which is sought to beinterrupted by the interrupter device may be completed by makingsuitable connections to contact 21, electrically and mechanicallymounted upon end member 12 and terminal 22, electrically andmechanically mounted upon rod 18. A suitable insulator shield, such asmetallic cylindrical member 23, capped with an arc-preventing ferrule 24is interposed between electrodes 15-16 and insulator 11 to prevent thelatter from becoming coated with metal.

The volume within interrupter chamber 10 is suitably evacuated throughan exhaust tubulation (not shown) during the final assembly thereof. Forproper operation of the interrupter as a vacuum-type interrupter ofalter nating currents, the pressure within chamber 10 must be maintainedat a pressure at least as low 10- mm. of mercury, but is preferablymaintained within the range of 10- to '10- mm. of mercury. The foregoingrequirement is essential for the operation of the devices as vacuuminterrupters of alternating currents. This requirement is necessarybecause, in order that the current-carrying arc struck betweenelectrodes 15 and 16 be extinguished at the first occurring current zerovalue, there must be substantially no ionizable gas present withinchamber 10. The occurrence of ionization may be substantially preventedif the possible breakdown paths between electrodes 15 and 16, or theirrespective supports, are small with respect to the mean free path of anelectron within the atmosphere obtained within the device. This meanfree path is designated as the statistical distance which an electronmay travel without colliding with a gas molecule at a given pressure.These conditions may be established in operative circuit interruptorsonly when the pressure within interrupter 10 chamber is below 10*' mm.of mercury and preferably below 10* mm. of mercury.

In FIG. 2 of the drawing there is shown, in graphical form, anillustration of the chopping phenomena. In FIG. 2, the instantaneousvalue ofa sinusoidal alternating current which is sought to beinterrupted bya vacuum circuit interrupter is plotted for one-halfcycle. As the current depicted by curve A rises from instantaneous valueof zero, the contacts, as for example contacts and 16 in FIG. 1, areseparated, at point B causing an arc dischargeto be establishedtherebetween. This are discharge is sustained exclusively by'the metalevaporated from contacts 15 and 16 by the heat generated at the contactsurfaces by the arc. The terminal points of the arc are knownrespectively as cathode and anode spots. As a matter of practice, mostof this evaporation occurs at the cathode, or negatively maintainedelectrode. It is, however, difiicult to predetermine which electrode isnegative at a particular instancewhen an alternating current circuit isinterrupted. In FIG. 2 the value of current inthe arcfollows its naturalcourse along the sinusoid of curve A and, for high current arcs (those;above 500 amperes) follows the dotted line until a zero value isreached. At this instant the arc is extinguished and the energizedmetallic ions between the electrodes rapidly ditfuse to the cold Wallsof members 12, 13 and 23 where they are cooled and deionized. The areremains extinguished because, when a high voltage builds up betweencontact 15 and 16, the arc is not reestablished, due to the highdielectric strength of the vacuum separating the contacts. 7

In the operation of vacuum circuit interrupters wherein the currentsought to be interrupted is of a relatively low value (below 500amperes) the instantaneous current value does not follow the dotted linecontinuously to a Zero value but, rather, at some low current value,denominated by 1 and occurring at a time 0, the arc is abruptly andprematurely extinguished. This results in an instantaneous change ofcurrent from a value of I to Zero. The value T is referred to herein asthe chopping current value for a particular device. As will be readilyappreciated, this almost instantaneous change of current from I to zeroresults in a high rate of change of current with time (di/dt) andresults in the production of extremely high surge voltages and inductiveloads which may be connected thereto. These surges may cause thebreakdown of insulation and are generally highly injurious to electricalequipment.

In the copending application T. H. Lee and J. D. Cobine, Serial Number750,784 filed concurrently herewith, it is disclosed that chopping oflow level alternating currents in vacuum circuit interrupters may becaused by an unstable condition resulting from a preponderance ofarcconstricting magnetic pressure over opposing vapor pressure in theregion of the electrodes between which the arc is struck. As a remedy tothe chopping problem, it is shown therein that vacuum circuitinterrupters may be constructed utilizing arc-electrodes comprisingparticular high vapor pressure materials, so that suflicient vaporpressure is available to counterbalance the arc-constricting magneticpressure.

Although the contact materials contemplated by the aforementioned Leeand Cobine application are quite satisfactory in providing low values ofchopping current in the interruption of low current alternatingcurrents, certain of these materials in elemental form tend toberelatively soft to have low melting points. As a result of thesecharacteristics, the elemental metals may be readily eroded and, whenmelted at thearc temperature, tend to weld and stick. Additionally,certain low melting point electrodes have an undue tendency to weldunder the heating effect of momentary currents before the arc-electrodesare separated and an arc isstruck.

In accord with the present invention at least one of electrodes 15 and16, and in some instances both of these electrodes, are composed ofintermetallic compounds at least one constituent of which is a metalhaving a vapor pressure lower than that of cadmium and higher than thatof lanthanum. The intermetallic compounds are further characterized ashaving melting points higher than 800 C. and being composed of metallicconstituents which are stable in air and which may be suitably processedby vacuum melting and similar techniques to provide the metal in pureform, suitable for the formation of a pure intermetallic compoundsuitable for use in the devices of the present invention. Therequirement that one of the constituents of the intermetallic compoundhave a vapor pressure between that of cadmium and lanthanum needs noelaboration. The requirement that each of the constituents of thecompound be stable in air may be de- 3 fined as requiring that degree ofstability sufiicient so that, when themetal is exposed for extendedlengths of time to atmospheric air at room temperature, no reactionoccurs between the metal and the atmosphere other than the formation ofa thin oxide coating, the order of afew molecules thick. Such coatingsare commonly formed by most known metals, even those regarded as themost stable in air. metallic compounds utilized in the present inventionbe suitable for processing vacuum melting or similar processes is simplythat the material be one whose. vaporpressure does not causevaporization or sublimation before melting in vacuum-melting, or similarprocesses, and one which is not so reactive that it cannot becontained'in a suitable vessel while vacuum processing is performed.

The requirement that the constituents of the interrather than metallic,characteristics.

I As it is well known to the art, an intermetallic compound, althoughfalling within the broad definitions of the term alloy, is actually abinary chemical compound between two metallic constituents wherein theproportion of the constituents occurs in fixed integral ratios andwherein a definite crystal lattice structure of quite complicated naturewhich has regular characteristics is formed. The characteristics of thecompound generally depend upon the nature and type of the crystallattice. Thus, for example, certain interrnetallic compounds (especiallythose between metals of groups 3 and 5 of the periodic table) possess acrystal lattice structure adapted to the complete filling of an energyband with electrons, so that the intermetallic compound actuallydisplays, semiconducting,

Although, in general, such intermetallic compounds are not suitable foruse as are electrodes or contacts in the devices of the presentinvention reference is made herein to them in order to emphasize theunique and unusual characteristics which intermetallic compounds, ingeneral, may possess.

The intermetallic compounds which I have found to be ideally suited forarc-electrodes or contacts in the vacuum circuit breakers of the presentinvention include: Cu Ce,

Cu La, Al Ce, Al La, AlSb, Bi Ce Bi Mg SnCe SnLa PbLa and MgSb.

Arc electrodes having portions thereof suitable to be utilized as theterminal points for an alternating current are to be interrupted invacuum made from the intermetallic compounds of the invention, exhibitthe advantages of the generic class of materials set forth in theaforementioned Lee and Cobine application. Additionally, theseinterrnetallic compounds generally possess unusually high meltingpoints, even as compared with either constituent from which they areformed. The desirability and the great utility in utlizing high meltingpoint materials for vacuum switch arc-electrodes are set forthhereinbefore. A further advantageous characteristic is that theintermetallic compounds utilized herein are quite brittle, without beingso brittle as to be so fragile that their use as contacts in a vacuumarc interrupter is precluded.

As an example of the unusually high melting points of the intermetalliccompounds utilized in accord with the present invention, Al Ce, althoughcomposed of constituents having melting points of 660 C. and 830 C.respectively, has a melting point of approximately 1465 C. Likewise, AlLa, although composed of constituents having a melting point 660 C. and812 C. respectively, exhibits a melting point of approximately 1424 C.AlSb, although composed of constituents having melting points of 660 C.,and 630 C. respectively, exhibits a melting point of approximately 1050C. Bi Ce although composed of constituents having melting points of 262C. and 830 C. respectively, exhibits a melting point of approximately1600 C. Bi Mg although composed of constituents having melting points of262 C. and 650 C. respectively, exhibits a melting point ofapproximately 823 C. SnCe although composed of constituents havingmelting points of 232 C. and 830 C. respectively, itself exhibits amelting point of 1400 C. SnLa although composed of constituents havingmelting points of 232 C. and 812 C. respectively exhibits a meltingpoint approximately 1420 C. PbLa although composed of constituentshaving melting points of 327 C. and 812 C. respectively, exhibits amelting point of approximately 1315 C. MgSb, although composed ofconstituents exhibiting melting points of 630 C. and 650 C.respectively, exhibits a melting point itself of 1228 C. In view of theforegoing, it may readily be seen that intermetallic compounds utilizedin the present invention possess unique and unexpected high-meltingpoint characteristics which suit them ideally for use as thearc-electrodes of the vacuum circuit interrupters of the presentinvention. To further illustrate the characteristics of theintermetallic compounds of thepresent invention as contrasted withcharacteristics of their constituents, reference is Table I VaporPressure at 800 K. (mm. s)

Electrode Material or Constituent 1\ Ieltin Point C."

That the melting point of vacuum alternating circuit current areinterrupters is important in preventing excessive melting in thecontacts and, more important, in preventing the contacts from weldingtogether, may readily be seen from fact that the cathode spot of avacuum arc interrupter, such as is utilized as to interrupt currentsseveral hundred amperes magnitude at 600 volts, often has a temperatureof 2500 K. to 3500 K.

A further unexpected advantage obtained in vacuum arc interrupterutilized for the interruption of alternating current in accordance ofthe present invention, by utilizing the intermetallic compounds setforth herein, is the highly desirable intermediate brittleness of thesecompounds as compared with other elements and alloys having otherdesirab'e characteristics. As is well known to the art, welding togetherof the contacts of the vacuum switch is a serious problem. If a weldbetween two contacts composed of a relatively ductile element, such asbismuth, does tend to form, the weld may withstand the initial shock ofan attempt to separate the contacts, so that a permanent juncturetherebetween isformed. The intermetallic compounds of this application,however, are all quite hard and brittle and, should an initial weldingaction tend to take place, this brittleness, with the attendent lack oftensile strength, allows for such an initial weld to be broken so thatthe contacts do not permanently join together. The brittleness of theintermetallic compounds of the present invention does not, however,approach the point, as does that of certain other elements and alloys,where shattering of the contacts becomes a problem.

As a condition precedent to satisfactory operation to any vacuum switch,it is necessary that the electrodes, between which an arc is momentarilyto exist, be substantially free of occluded and sorbed gases. Thisrequires that the electrodes be previously conditioned in order toremove from them all sorbed and occluded gases. For this reason, amongothers, commercially useful arc vacuum circuit interrupters have notheretofore been constructed utilizing any of the materials whichcomprise the intermetallic compounds of the present invention. This isprimarily due to the fact that rendering a material free of occluded andsorbed gases is most ode electrode, another choppmgphenomenoneffectively done by out-gasing and baking the material at an extremelyhigh temperature. Thus, commercially available vacuum circuitinterrupters have, in the past, utilized refractory metals as theelectrodes thereof since these materials may be heated to temperaturesof the order of 2000 C. without substantial evaporating or melting.Obviously, such treatment cannot be utilized upon the intermetalliccompounds utilized in the circuit interrupters of the present invention.However, it has been found that, once certain metals have been freed ofoccluded and sorbed gases by suitable treatment, the material may beexposed to atmospheric or higher pressures of air or other gases withoutbecoming contaminated other than by the formation of a thin protectiveoxide over the surface thereof generally of the order of molecules inthickness. Accordingly, the electrodes of the present invention may beformed by first removing occluded and sorbed gases from the constituentof the intermetallic compounds individually in their metallic state asfor example, by repeated arc-meltings'in a vacuum. The constituents arethen carefully measured to obtain stoichiometric proportions to form thedesired compound and are once again melted and mixed in vacuo and castin the desired electrode form.

It is, of course, necessary to provide a standard for 0ccluded andsorbed gases for Vacuum interrupter contacts in order that theinterrupter be operative. Such a standard may be set analytically in therequirement that the final arc electrode be of such purity as to containless than 10* atomic parts of all gases. As a practical mat ter, thisrequirement may further be attainedif contacts utilized in the presentinvention, when placed in a vacuumized test chamber a few liters involume and subse quently deeply eroded by a repetitive arcing, as forexample, with the voltage of commercial power at a cur- 4 rent of 100amperes or more, the pressure level of the container, a few cycles afterarcing, does not rise substantially from its initial value in theabsence of getters and pumps, even if the initial pressure is of theorder of 10- mm. of mercury. Since repeated arc erosion, in accord withsuch a test, consumes a substantial proportion of the contact itprovides a satisfactory test for the determination whether thearc-eectrodes are substantially free of occluded and sorbed gases, sinceeven the presence of an infinitesimal amount thereof would cause asubstantial rise in pressure of the test chamber.

After the arc-electrodes constructed of the intermetallic compounds setforth herein have been prepared in accord with the foregoing proceduresand representative samples thereof properly tested, the electrodes areassembled into the interrupter device as illustrated in FIG. 1 and theentire device is connected to a suitable vacuum pump and evacuated.While being evacuated to a pressure of at least mm. of mercury or less,the entire device is suitably raised to a temperature of at least 500 C.in order to bake out all of the components thereof to free them fromsurface adsorbed gases so that an operative pressure may be maintainedtherein, even under high temperature conditions caused by repeatedarcing be tween the electrodes thereof. Suitably, the device may beevacuated for approximately ten hours a pressure of 10- mm. of mercury.After the foregoing treatments, the tubulation connecting theinterrupter chamber to the evacuation means is closed off, and thedevice is ready for use as a vacuum c rcuit nterrupter for alternatingcurrents of low magnitude to provide low chopping values.

Although one object of the present invention is to provide improvedchopping values for low voltage alternating current vacuum circuitinterrupters by the provision of high melting point, intermediatebrittleness, interme-tallic compound contacts to avoid vapor starvationat the cathfactor which can contribute to the is the presence ofinstabilities at the electrodes. It has been proposed to remove thesein- 8 stabilities by utilizing, as the electrodes of the vacuum circuitbreaker, a material having a low-work function, Accordingly, in accordwith a preferredembodiment of the present invention, while oneconstituent of the intermetallic compound comprising the arc electrodesof the vacuum circuit breakers of the invention comprises a metal havinga vapor pressure between that of lanthanum and cadmium and satisfyingcertain other conditions, the other material comprising theintermetallic compound is chosen from the metals exhibiting awork-function below 3.5 e.v. as for example, cerium or lanthanum. Thisconstituent must, of course, also be stable in air and suitable forvacuum processing. Accordingly, while allof the intermetallic compoundsidentified hereinbefore provide superior arc-electrodes for vacuum arcinterrupter-s in accord with the present invention, certain of thesematerials namely, Al Ce, Al La, Bi ce SnCe and PbLa provide addedinsurance against the premature extinguishment of an alternating currentarcby virtue of the incorporation therein of a material having alow-Work function namely, cerium or lanthanum. Additionally, althoughthe compounds Cu Ce and Cu La have melting points lower than purecopper, they are suitable as arc-electrodes 7 due to the low-workfunctions of cerium and lanthanum. Thus, for example, a vacuuminterrupter satisfying the criteria set forth hereinbefore and havingarc-electrodes of Cu ce held the chopping current to a value of 1.75 A.

at low current values and successfully and repeatedly interruptedcurrents of 6500 A. at 15000 volts.

Although it is an object of the present invention to provide harder,intermediately brittle, and high melting point, highvapor pressure,contact electrodes for vacuum circuit interrupters utilized foralternating current circuit interruption, and the intermetalliccompounds of the present invention serve this object, in certaininstances greater mechanical strength than may be supplied by thematerials themselves, is required. Accordingly, while the contacts ofcircuit interrupters constructed in accord with the present inventionmay be composed entirely of these intermetallic compounds, theseintermetallic compounds may also be used as filler substances to fillthe pores of a porous refractory matrix, as for example, tungsten,molybdenum or the carbides thereof. In accordwith this embodiment of theinvention the porous matrix is prepared, as for example, by sinteringtungsten o-r molybdenum powder at high temperature and pressure. Thevacuum-treated intermetallic compound is then infiltrated into theporous mass at a suitable high temperature in vacuo. In such anarrangement, the mechanical strength of the porous refractory matrixadds greatly to the ruggedness and long length characteristics of thecircuit interrupter.

As is mentioned hereinbefore, presently available evidence indicatesthat chopping, the premature extinction of an alternating current arcbefore a normally occurring current zero, is primarily a phenomena whichoccurs at the cathode electrode. Accordingly, when suitable precautionsare taken so that a given electrode may be made the instantaneouscathode when an alternating current is interrupted, in order that theadvantages of the present invention be obtained. it is only necessary,in the instance of an alternating current interrupter with which theseprecautions are taken, that the in-termetallic compounds of theinvention be used for the cathode electrode. Accordingly, it is withinthe scope of the present invention that operative vacuum circuitinterrupter which exhibits low values of chopping currents Withoutundesirable welding or sticking characteristics may be constructedwherein only one of the arc-electrodes is fabricated of theintermetallic compounds of this invention. It is, however, contemplatedin a preferred embodiment of the invention that both electrodes beconstructed of these compounds, since under certain circumstances, it isdifficult to ascertain which eletrode Will be the cathode at the instantthe circuit is interrupted and it may not be feasible to provide meansfor establishing such certainty.

While the invention has been disclosed herein with l respect to certainembodiments thereof, it is apparent that many modifications and changeswill readily occur to those skilled in the art. Accordingly, by theappended claims I intend to cover all such modifications and changes asfall within true spirit and scope of the present invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A vacuum alternating current circuit interrupter comprising; anevacuable envelope evacuated to a pressure lower than mm. of mercury; 21pair of electrical terminals adapted for connection in an alternatingcurrent circuit; a pair of electrodes located Within said envelope,connected in circuit between said terminals and disposed in spaced-apartrelationship during circuit interrupting operation to allow for theestablishment of a circuit interrupting arc uherebetween, each of saidelectrodes having a region on which a respective elect-rode spot for alow current are may be established; one of said electrode regiOns beingformed of an intermetallic compound having a melting point in excess of800 C., at least one constituent of which comprises a metal having avapor pressure greater than that of lanthanum and less than that ofcadmium, all constituents of said compound being stable in air andsuitable for processing in vacuo to remove gaseous impurities therefrom.

2. A vacuum alternating current circuit interrupter comprising; anevacuable envelope evacuated to a pressure lower than 10* mm. ofmercury; a pair of electrical terminals adapted for connection in analternating cur-rent circuit; a pair of electrodes located within saidenvelope, connected in circuit between said terminals and disposed inspaced-apart relationship during circuit interrupting operation to allowfor the establishment of a circuit interrupting arc therebetween, eachof said electrodes having a region on which a respective electrode spotfor a low current are may be established; one or" said electrode regionsbeing formed of an intermetallic compound having a melting point inexcess of 800 C., at least one constituent of which comprises a metalhaving a vapor pressure greater than that of lanthanum and less thanthat of cadmium, another constituent of said compound being a metalhaving a work function less than 3.5 electron volts, all constituents ofsaid compound being stable in air and suitable for processing in vacuoto remove gaseous impurities therefrom.

3. The circuit interrupter of claim 1 wherein the electrode regioncomprises an intermetallic compound seleoted from the group consistingof Al Ce, Al La, AlSb, Bi Ce Bi Mg sncez, snLaz, PbLa Sn CC, Sn La, CuCe and Cu La.

4. The circuit interrupter of claim 1 wherein the electrode region is AICe.

5. The circuit interrupter of claim 1 wherein the electrode region is AlLa.

6. The circuit interrupter of claim 1 wherein the electrode region isA181).

7. The circuit interrupter of claim 1 wherein the electrode region is BiCe 8. The circuit interrupter of claim 1 wherein the electrode region isBi Mg 9. The circuit interrupter of claim 1 wherein the electrode regionis SnCe 10. The circuit interrupter of claim 1 wherein the electroderegion is SnLa 11. The circuit interrupter of claim 1 wherein theelectrode region is PbLa 12. The circuit interrupter of claim troderegion is MgSb.

13. The circuit interrupter of claim trode region is Sn C'e.

14. The circuit interrupter of claim trode region is Sn La.

15. The circuit interrupter of cl aim trode region is Cu Ce.

16. The circuit interrupter of claim trode region is Cu La.

17. The circuit interrupter of claim work function material is cerium.

1 wherein the elec- 1 wherein the elec- 1 wherein the elec- 1 whereinthe elec- 1 wherein the elec- 2 wherein the low- 18. The circuitinterrupter of claim 2 wherein the lowwork function material islanthanum.

References Cited in the file of this patent UNITED STATES PATENTS1,334,150 Green et al. Mar. 16, 1920 1,784,303 Millikan et al. Dec. 9,1930 1,901,639 Eschholz Mar. 14, 1933 2,049,500 Hensel Aug. 4, 19362,121,180 Vatter June 21, 1938 2,156,974 Doan May 2, 1939 2,419,469Spiro Apr. 22, 1947 2,888,741 Knapp et al. June 2, 1959 2,900,476 ReeceAug. 18, 1959 FOREIGN PATENTS 153,306 Great Britain Sept. 29, 1921

